1. Field of the Invention
The present invention relates to extreme ultraviolet (EUV) lithography, and particularly to an EUV radiation source configured for transmitting an improved EUV beam.
2. Discussion of the Related Art
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems operating around 248 nm, as well as the following generation of ArF-excimer laser systems operating around 193 nm. Industrial applications in the Vacuum UV (VUV) range involve the use of the F2-laser operating around 157 nm. EUV radiation sources for EUV lithography emitting 11-15 nm photon beams are currently being developed.
EUV radiation sources have an advantageous output emission beam including 11-15 nm wavelength photons having photon energies greater than 90 eV. This short wavelength is advantageous for industrial applications, such as particularly photolithography, mask writing and mask and wafer inspection applications, because the critical dimension (CD), which represents the smallest resolvable feature size producible using photolithography, is proportional to the wavelength. This permits smaller and faster microprocessors and larger capacity DRAMs in a smaller package.
A promising technique for producing EUV lithography beams use a pair of plasma pinch electrodes for driving a preionized azimuthally symmetrical plasma shell to collapse to a central axis. A power supply circuit supplies a high energy, short duration pulse to the electrodes, wherein several kilovolts and up to 100 kiloAmps are applied over a pulse duration of less than a microsecond. A Z-pinch electrode arrangement generates a current through the plasma shell in an axial direction producing an azimuthal magnetic field that provides the radial force on the charged particles of the plasma responsible for the rapid collapse.
The excimer and molecular fluorine lithography lasers, mentioned above, emit laser beams using a gas discharge for creating a population inversion to a metastable state in the laser active gas, and a resonator for facilitating stimulated emission. It is not yet clear what radiative mechanism is responsible for the axial, high energy photon emission in plasma pinch EUV sources. The collapsing shell of charged particles of the plasma have a high kinetic energy due to their velocities in the radial direction. The rapid collapse of the shell results in collisions between all portions of the incoming shell at the central axis with radially opposed portions of the incoming shell.
The high kinetic energies of the particles are abruptly transformed into a hot, dense plasma which emits x-rays. A high recombination rate concentrated in the azimuthal direction due to the plasma being particularly optically dense in the azimuthal direction has been proposed (see, Malcolm McGeoch, Radio Frequency Preionized Xenon Z-Pinch Source for Extreme Ultraviolet Lithography, Applied Optics, Vol. 37, No. 9 (20 Mar. 1998), which is hereby incorporated by reference), and population inversion resulting in spontaneous emission and predominantly axial stimulated emission, and bremsstrahlung resulting from the rapid radially deceleration of the charged particles of the collapsing plasma, are other mechanisms of high energy photon emission.
It is desired to have an improved EUV photon source, particularly having output emission characteristics more suitable for industrial applications such as photolithography.
In view of the above, an EUV photon source is provided including a plasma chamber filled with a gas mixture, multiple electrodes within the plasma chamber defining a plasma region and a central axis, a power supply circuit connected to the electrodes for delivering a main pulse to the electrodes for energizing the plasma around the central axis to produce an EUV beam output, and a preionizer for ionizing the gas mixture in preparing to form a dense plasma around the central axis upon application of the main pulse from the power supply circuit to the electrodes.
According to a first embodiment, an ionization unit is positioned along a beam path of the EUV beam outside of the plasma region for ionizing contaminant particulates along the beam path. An electrostatic particle filter is further provided for collecting the ionized particulates. The ionizing device may be preferably of corona-type.
According to a second embodiment, one or more, and preferably a set of, baffles is disposed along a beam path outside of the pinch region. The baffle(s) may function to diffuse gaseous and contaminant particulate flow emanating from the pinch region. The baffle(s) may also function to absorb or reflect acoustic waves emanating from the pinch region away from the pinch region.
According to a third embodiment, a clipping aperture is disposed along a beam path outside of the pinch region for at least partially defining an acceptance angle of the EUV beam. The aperture may be formed of ceramic and may particularly be formed of Al2O3.
According to a fourth embodiment, the power supply circuit generates the main pulse and a relatively low energy prepulse before the main pulse for homogenizing the preionized plasma prior to the main pulse.
According to a fifth embodiment, a multi-layer EUV mirror is disposed opposite a beam output side of the pinch region for reflecting radiation in a direction of the beam output side for output along the beam path of the EUV beam. The EUV mirror preferably has a curved contour for substantially collimating or focusing the reflected radiation. In particular, the EUV mirror may preferably have a hyperbolic contour.